Elstree to St. John's Wood Cable Tunnel

The Elstree to St. John's Wood Cable Tunnel, known as The London Connection during construction, is a 20 km long, 3m wide tunnel beneath northwest London. Constructed between 2001 and 2005, the tunnel carries high voltage transmission lines from Elstree substation in Hertfordshire to Lodge Road sub station in Westminster at a depth of 20m below street level.^[1]

Elstree to St. John's Wood Cable Tunnel
Location
Country	UK
Province	Greater London
Ownership information
Owner	National Grid plc
Construction information
Construction started	2001
Commissioned	2005
Technical information
Type	Underground tunnel
Type of current	AC
Total length	20 km (12 mi)
AC voltage	400 kV

The tunnel runs beneath the A5 road for the majority of its length, and houses a single 400 kV power transmission circuit with a rated capacity of 3700 A. There is provision for a second circuit to be installed in the future.^[2]

A remotely operated, battery powered monorail system runs along the tunnel, allowing the cables to be inspected remotely using infrared cameras.^[3]

Route of Elstree to St John's Wood cable tunnel